There are lots of different themes you could use. A
Pumkin Festival would be fun, the kids could decorate pumpkins (either real or paper), have a pumpkin "hunt and find" (sort of like an Easter egg hunt, but with miniture pumpkins), and serve treats like pumpkin bread or muffins.
You could also do a broader
Harvest Festival theme that incorporates other traditional activites like apple bobbing, making cereal necklaces, and crafting corn dolls.
here are some sites with more ideas:
Harvest Party Ideas - Halloween Alternatives
Kids' Harvest Party Games and Ideas